Alternating current (AC) motors are used in a variety of applications, including vehicle applications, and are desirable for having a simple, rugged construction, easy maintenance, and cost-effective pricing. The AC motors used in vehicle applications are typically controlled via a voltage source inverter such that the motor phase currents are sinusoidal. Supplying a sinusoidal-shaped input current to the AC motor typically produces the highest average torque without additional low-frequency harmonics, which can be a source of torque pulsations in AC motors. In vehicle applications, one design consideration is to maximize the utilization of the available direct current (DC) voltage (e.g., provided by a battery). Maximization of the voltage utilization generally improves the high speed power and overall system efficiency.
Some AC motors are permanent magnet (PM) machines, which typically have high power density and high efficiency characteristics and are thus well-suited for vehicle propulsion applications PM machines typically include a PM motor driven either directly or through a belt from the prime mover of the vehicle, a diode rectifier coupled to the PM motor, and a switch coupled to the output of the diode rectifier and the vehicle's battery to control the flow of current to the battery during operation of the vehicle. In current PM machines, the switch is generally a silicon-based semiconductor device (e.g., an insulated gate bipolar transistor (IGBT) or a metal oxide field effect transistor (MOSFET)) that is normally OFF.
In operation as an alternator, PM machines act as a constant current source, while the switch mode rectifier acts to control the voltage and power to the battery when the PM machines are operating above a threshold speed. To act as a constant current source, the PM motor is typically designed with a back electromotive force (EMF) approaching at least 4-10 times more than that of the battery output voltage. During operation of the PM machine, current normally flows to the battery since the switch is normally OFF. To divert current from flowing to the battery, the switch is turned ON which causes current to flow through the switch and back to the diode rectifier instead of to the battery.
When operating with the appropriate controlled pulse width modulation ON/OFF switching action of the switch, the battery is exposed to its appropriate average controlled voltage. In the event of a control error or other error which results in the lack of switching action of the switch, the battery will be exposed to the rectified back EMF of the PM motor. Since the rectified back EMF of the PM motor is greater than the controlled battery voltage when the PM motor is producing its normal output, the battery will be exposed to a larger average voltage than during normal operation.
Accordingly, it is desirable to provide an alternator, system, and method that reduce a potentially damaging high voltage fault condition. Additionally, it is desirable to provide an alternator, system, and method for providing a low-cost, high power alternator application. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.